Gamma-compensated ionization chamber



March Z1, 1961 L. E. JOHNSON GAmnA-con/IPENSATED IONIZATION CHAMBER 2Sheets-Sheet 1 Filed Nov. 22, 1957 w WVNNWNWHNNMV NW m N v NN ESL [E ralf/ferri? Sgt@ March 21, 1961 L. E. JOHNSON GArvuwA-coMPENsATEDIoNIzATIoN CHAMBER 2 Sheets-Sheet 2 Filed Nov. 22, 1957 INVENTOR. 5.c/O/f/VSO/V,

GAMlVIA-CUMPENSATED INIZATION CHAMBER Leslie E. Johnson, 168th andKilpatrick Sts., RR. 1, Tinley Park, Ill.

Filed Nov. 22, 1957, Ser. No. 698,226

12 Claims. (Cl. 25th-183.1)

This invention relates to ionization chambers, and more particularly toan ionization chamber highly sensitive to neutrons and provided withmeans for compensating for response to gamma radiation.

A main object of the invention is to provide a novel and improvedionization chamber especially adapted for the detection of neutrons andintended for use over a wide range of temperatures, the ionizationchamber being relatively simple in construction, being relativelycompact in size, and being arranged so that the effects of gammaradiation are substantially eliminated and the response of the chamberwill be limited essentially to response to neutrons.

A further object of the invention is to provide an improved ionizationchamber adapted for use at relatively high temperatures and highlysensitive to the presence of neutrons, the chamber being provided withmeans for substantially eliminating the effects of gamma radiation, thechamber being relatively inexpensive to manufacture, being durable inconstruction, and involving relatively few parts.

A still further object of the invention is to provide an improved methodof eliminating the effects of gamma radiation in quantitativelymeasuring the intensity of neutron radiation.

A still further object of the invention is to provide an improved systemfor quantitatively measuring the intensity of neutron radiation byemploying ionization chambers utilizing neutron-sensitive gas fillings,the system being arranged to compensate for the presence of gammaradiation by the proper selection of the neutron-sensitive gas fillingsin the respective ionization chambers.

Further objects and advantages of the invention will become apparentfrom the following description and claims, and from the accompanyingdrawings, wherein:

Figure l is a longitudinal vertical cross sectional 'View taken througha twin ionization chamber assembly constructed in accordance with thepresent invention.

Figure 2 is an end view of the ionization chamber assembly of Figure 1,seen from the terminal end of the assembly.

Figure 3 is a transverse vertical cross sectional View taken on the line3 3 of Figure l.

Figure 4 is a transvesre vertical cross sectional view taken on the lineie-4 of Figure l.

Figure 5 is a transverse vertical cross sectional view taken on the line5-5 of Figure l.

Figure 6 is a transverse vertical cross sectional view taken on the line6-6 of Figure 1.

Referring tothe drawings, 11 generally designates an ionization chamberassembly constructed in accordance with the present invention. Theassembly 11 comprises an elongated metal outer casing or housing 12provided with a rst end wall 13 and with a second end wall 14, the endwall 13 having the peripheral ange 15 which is received in and sealinglysecured to the inside surface of one end portion of the housing 12, andthe end wall 14 being integrally formed in a liner 16 which is received'YZ-,976,418 Patented Mar. 2i, 1961 in and sealingly secured to theopposite end of housing 12.

Secured in the casing 12 adjacent end wall 13 is a liner member 17having the inner transverse wall 18. Similarly, the liner member 16 hasan inner transverse wall 19.

Both of the liner members 17 and 16 are of metal and are thuselectrically connected to the metal outer casing 12.

Designated respectively at 20 and 21 are a pair of ionization chamberswhich are provided in the main housing `12, the ionization chamber 20comprising an elongated hollow electrode 22 which is axially mountedwithin a cylindrical sleeve electrode 23. The electrode 22 constitutesthe signal electrode, and the sleeve electrode 23 is the element of thechamber to which the high energizing voltage is applied. Similarly, theionization chamber 21 comprises an elongated hollow signal electrode 24and a cylindrical sleeve electrode 25 in which the signal electrode 24is axially disposed.

As shown in Figure l, the cylindrical sleeve electrodes 23 and 25 aresupported at their ends by the respective transverse wall elements 18and 19 and are electrically insulated therefrom by suitable sleevemembers 26, 27, and 28, 29 of refractory insulating material, such asalumina or the like, of a nature capable of withstanding hightemperature conditions. The ends of the cylindrical electrodes 23 and 25are also sealingly connected to the transverse wall elements `19 and 18through the refractory insulating sleeves 26, 27, 28 and 29 throughsuitable metal connecting collars, such as the collars 30 and 31, 32 and33, 34 and 35, and 36 and 37, shown in Figure l. The collar elements 31,33, 34 and 36 are fastened to the peripheral portions of respectiveannular ring elements 38, 39, 40 and 41. Rigidly secured to the ringelements and electrically connected thereto are the respective inwardlyextending metal guard sleeves 42, 43, 44 and 45 which are arrangedcoaxially around the signal electrodes 22 and 24 and which projectinwardly substantial distances within the end portions of the respectivecylindrical electrodes 23 and 25, whereby the end portions of thecylindrical electrodes overlap the inner ends of the guard sleeves.

The guard sleeves 42 to 45 are thus electrically connected to the outercasing 12 through the ring members 38 to 41 and through the associatedtransverse wall elements 18 and 19.

As shown in Figure l, the signal electrodes 22 and 24 projectsubstantial distances on opposite sides beyond the transverse wallelements 1S and 19. The ends of the signal electrodes adjacent the wallIelement 1S are supported therefrom by means of refractory insulatingsleeves 44 and 45. The sleeves 44 and 45 are sealingly connected to theannular ring elements 38 and 39 by metal collar or sleeve members 46 and47. The opposite ends of the insulating sleeves 44 and 45 are sealinglyconnected to the end portions of the signal electrodes 22 and 24 byannular metal cup elements 48 and 49.

The end portions of the signal electrodes 22 and 24 are secured to thetransverse wall element 19 through refractory insulating sleeves 50 and51 which are sealingly secured between the transverse wall element 19and the respective end portions of the signal electrodes 22 and 24. Thusthe refractory insulating sleeve 50 is sealingly secured at one end tothe annular ring member 40 by a metal collar S2, whereas the oppositeend of the insulating sleeve 50 is sealingly secured to the signalelectrode 22 by an annular metal cup member 53. In the same manner, oneend of the sleeve 51 is sealingly secured to the annular ring member 41by a metal collar 54 and the opposite end of the insulating sleeve 51 issealingly secured to the signal electrode 24 by an annular cup member55.

The hollow tubular electrodes 22 and 24 are formed with apertures 22'and 24' adjacent the insulating sleeves 27 and 29. The electrodes areprovided with metal sealing` plugs -56 and :57 at the ends thereofadjacent the.v

casing end wall 13 and are provided' at their opposite ends with fillingcaps 58 and 59 for admitting ionizable gas .into the chambers, said capsbeing subsequently crimped, as shown.

Mounted in the end wall 14 and suitably insulated therefrom are therespective signal terminal prongs 60 and 61 and the respective highvoltage terminal prongs 62 and 63. The terminal prongs are sealinglysecured to the end Wall 14 and insulated therefrom by insulating sleeves64, 65, 66 and 67 of suitable refractory insulating material, such asalumina, or the like, having the ability to withstand high temperatureconditions.

The inner end of the signal terminal prong 60 is electrically connectedto signal electrode 22 by a conductor 68 and its associatedcontact clamp69, and the signal terminal prong 61 is smilarly electrically connectedto the signal electrode 24 by a conductor 7d' and its associated contactclamp 71. The inner end of the high voltage terminal prong 62 isconnected by a conductor 73' to the adjacent end of cylindricalelectrode 23, and the inner end of the high voltage terminal prong 63 isconnected by a conductor 74 to the end of the cylindrical electrode 25,as shown in Figure l; l Y f The end wall'li is provided with a suitablefilling tuber75for admitting an inert gas sudh as dry nitrogen, or thelike, into the main housing 12.

Main housing 12 is grounded or is placed at any other suitable referencepotential desired.

It will be understood that the filling tube 75 is suitably crimped andhard soldered after the inert gas has been introduced into the mainhousing, so that the gas will be thereafter retained in the housing. Y

The casingrlzis filled with a suitable inert gas, such as dry nitrogen,or the like, which thus surrounds the ionization chambers 2li and 2l.The chambers 2t) and 21 areliilled with respective gaseous materialshaving different neutron sensitivities but having substantially equalgamma sensitivities; Thus, ionization chamber 20 may be filled with -agaseous material having low neutron sensitivity, whereas the chamber 21may be filled with a gaseous material having high neutron sensitivity,the gaseous materials being selected to Vhave substantially equal gammasensitivities. For example, the chamber 2t) may be filled with a mixtureof gaseous compounds such as a mixture ofthe fluorides of differentisotopes of boron, such as a mixture of the B1o liuoride and' the B11-iluoride, for example, a mixture comprising 11% of the B11l fluorideand 89% of the B11 fluoride, said mixture having relatively low neutronsensitivity because of the relatively low percentage of the B iiuoride.The chamberzl is iilled with a gaseous material having relatively highneutron sensitivity, for example, a mixture of the same boron isotopefluorides but in different proportion. f Forexample, the gaseous mixturein the ionization chamber 21 may comprise 96% B10 fluoride and 4% B11fluoride, whereby the gaseous mixture in the chamber 21 has relativelyhigh neutron sensitivity. However, the gaseous mixtures in both of thechambers Ztl and 21 have substantially equal -garnma sensitivities,whereby the signal electrodes 22 and 24.* receive a substantially equalresponse to a given levelof gamma radiation, but receive differentresponses to the presence of a given number of neutrons.

Respective high voltages of different polarities with respect to theground or reference potential of casing 12 are applied to the respectivehigh voltage terminal prongs 62 and 63. For example, a positive highvoltage is applied to the terminal prong 62 and a negative high terminal74, whereby the signal responses of electrodes 22 and '24 are combined.Since the responses are opposite in polarity, the components of saidresponses derived from gamma radiation Will substantially cancel eachother and the net signal available at the output terminal '74 will thenbe in accordance-with the difference between the neutron responses ofthe signal electrodes 24 and 22. Thus, the response of signal electrode22 to neutrons is subtracted from the neutron response of the signalelectrode 24, since these electrodes are electrically connected togetherby the wires 72 and 73, providing a resultant signal at the outputterminal 74 which will Vbe in accordance with the number of neutronspresent.

'As will be readily apparent, the method herein disclosed foreliminating the effects of gamma radiation does not require lthat theionization chambers be located in the same housing, but the method maybe carried out in any system provided with twin ionization chambershaving gaseous fillings of different neutron sensitivity andsubstantially equal gamma sensitivity, even though the chambers bephysically separated, as long as their outer casings are at the same.reference potential and maintained in a substantially equal gamma iiux.It

will also be apparent that any suitable mixture of gases y maybeemployed as long as the gases have different neutron sensitivities andsubstantially equal gamma sensitivities.

It will be further understood that the high voltages applied to theterminal prongs 62 and 63 are of sub-Y stantially the same magnitude,although of opposite polarity with respect to the reference potential onthe l. in a system for measuring neutrons, a iirst ionizal tion chamberhaving a relatively high yneutron sensitivity,

. signal electrode, said chambers having substantially equal voltage is'applied to the terminal prong 63. The device will work equally wellwith the potentials reversed.

The signal output terminalv prongs 60 and 61 are connected by respectivewires 72 and 73 to a common signal a second ionization chamber having arelatively low neutron sensitivity, eachiof said chamberscomprising asignal electrode and a sleeve electrode surrounding said sensitivitiesto gamma radiation, said chambers being enclosed in a commonconductive'housing, a iilling oi inert gas in said common housing, meansapplying a high positive volt-age toV one sleeve electrode, meansapplying a high negative voltage to the other sleeve 'electrede, and`means connecting the signal electrodes together, whereby the eiects ofgamma'radiation `on the chambers are substantially canceledY andavresultant signal is provided at the connected signal electrodes inaccordance with the diier-ence 'between the responses of the chambers tothe presence of neutrons.

2. In a system for measuring neutrons,. a first ionization chambercontaining a mixture of gaseous isotope compounds, one of which hasahigh neutron sensitivity andthe other of which has low neutronsensitivity, ya second ionization chamber containing a mixture ofgaseous isotope compounds one of which has high neutron sensitivity andthe other of which has low neutron sensitivity, each of said chamberscomprising a signal electrode and a sleeve electrode surrounding saidsignal electrode, the mixtures of gaseous isotope compounds havingsubstantially equal sensitivities to gamma radiation, the mixture in oneof the chambers having a substantially greater percentageof `highneutron sensitivity gaseous isotope compound than ythe mixture in theother chamber, said chambers having common outer conductive (housingmeans at a common electrical potential, a filling of inert gas in saidcommon conductive housing means, means applying a high positive voltageto one sleeve electrode, means applying a high negative voltage to theother sleeve electrode, and means connecting the signal electrodestogether, whereby the effects of gamma radiation on the chambers aresubstantially canceled and a resultant signal is provided at theconnected signal electrodes in accordance with the difference betweenthe responses of the chambers to the presence of neutrons.

3. `In a system for measuring neutrons, a first ionization chambercontaining a mixture of fluorides of boron isotopes, one of which has ahigh neutron sensitivity and the other of which has low neutronsensitivity, a second ionization chamber containing a mixture ofiiuorides of boron isotopes one of which has high neutron sensitivityand the other of which has low neutron sensitivity, each of saidVchambers comprising a signal electrode andV a sleeve electrodesurrounding said signal electrode, the mixtures of boron isotopefluorides having substantially equal sensitivities to gamma radiation,the mixture in one of the chambers having a substantially greaterpercentage of high neutron-sensitivity boron isotope fluoride than themixture in the other chamber, said chambers having common outerconductive housing means at `a common electrical potential, a filling ofinert gas in said common conductive housing means, means applying a highpositive voltage to one sleeve electrode, means applying a high negativevoltage to the other sleeve electrode, and means connecting the signalelectrodes together, whereby the elects of gamma radiation on thechambers are substantially canceled and a resultant signal is providedat the connected signal electrodes in accordance with the differencebetween the responses of the chambers to the presence of neutrons.

4. In a system for measuring neutrons, a first ionization chamber filledwith a mixture of uorides of boron isotopes including the fluoride of Bisotope in a relatively high proportion, a second ionization chamberlled with a mixture of liuc-rides of boron isotopes including thefluoride of B10 isotope in a relatively low proportion, each of saidchambers comprising a signal electrode and a sleeve electrodesurrounding said signal electrode, said fluoride mixture havingsubstantially equal sensitivities to gamma radiation, said chambershaving common outer conductive housing means at a common electricalpotential, a iilling of inert gas in said common conductive housingmeans, means applying high potentials relative to said common potentialof opposite polarity to said sleeve electrodes, and means connectingsaid signal electrodes together, whereby a resultant signal is obtainedat the connected signal electrodes in accordance with the presence ofneutrons and wherein the responses to gamma radiation are substantiallycanceled.

5. A neutron measuring device comprising a sealed main conductivehousing, la pair of sealed ionization chambers in said housing, eachionization chamber comprising a tubular signal electrode, a sleeveelectrode surrounding the signal electrode, and insulating sleeve meanssealingly connecting the ends of the sleeve electrode to the endportions of the signal electrode, first ionizable gaseous material inone of the chambers, second ionizable gaseous material in the otherchamber, said gaseous materials having substantially equal sensitivitiesto gamma radiation but having substantially different sensitivities toneutrons, a filling of inert gas in said main housing surrounding theionization chambers and isolated from the gaseous material in theionization chambers, means applying high voltages of opposite polarityto said sleeve electrodes, and means connecting the signal electrodestogether, whereby the eiects of gamma radiation on the chambers aresubstantially canceled, and whereby a resultant signal is provided atthe connected signal electrodes in accordance with the diierence betweenthe responses of the chambers to the presence of neutrons.

6. A neutron measuring device comprising a sealed applying high voltagesof opposite'polarity-to said sleeveY Y' main conductive housing, a pairof sealed ionization chambers in said housing, each ionization chambercomprising an elongated signal electrode, a sleeve electrode surroundingthe signal electrode, insulating sleeve means sealingly connecting theends of the sleeve electrode to the end portions of the signalelectrode, respective conductive guard sleeves mounted in the endportions of the chamber concentrically `between the insulating sleevemeans and the signal electrode and projecting substantial distances intothe end portions of the sleeve electrode, and means electricallyconnecting the guard sleeves to the main housing, iirst ionizablegaseous material in one of the chambers, second ionizable gaseousmaterial in the other chamber, said gaseous materials havingsubstantially equal sensitivities to gamma radiation but havingsubstantially different sensitivities to neutrons, means electrodes, andmeans connecting the signal electrodes together, whereby the effects ofgamma radiation on the chambers are substantially canceled, and wherebya resultant signal is provided at the connected signal electrodes inaccordance with the difference between the responses of the chambers tothe presence of neutrons.

7. A neutron measuring device comprising a sealed main conductivehousing, a pair of sealed ionization chambers in said housing, eachionization chamber comprising an elongated signal electrode, a sleeveelectrode surrounding the signal electrode, insulating sleeve meanssealingly connecting the ends of the sleeve electrode to the endportions of the signal electrode, respective conductive guard sleevesmounted in the end portions of the chamber concentrically between theinsulating sleeve means and the signal electrode and projectingsubstantial distances into the end portions of the sleeve electrode, andmeans electrically connecting the guard sleeves to the main housing,first ionizable gaseous material in one of the chambers, secondionizable gaseous material in the other chamber, said vgaseous materialshaving substantially equal sensitivities to gamma radiation but havingsubstantially different sensitivities to neutrons, a filling of inertgas in said main housing surrounding the ionization chambers and beingisolated from the gaseous materials in the ionization chambers, meansapplying high voltages of opposite polarity to said sleeve electrodes,and means connecting the signal electrodes together, whereby the effectsof gamma radiation on the chambers are substantially canceled, andwhereby a-resultant signal is provided at the connected signalelectrodes in accordance with the difference between the responses ofthe chambers to the presence of neutrons.

8. A neutron measuring device comprising a sealed main conductivehousing, a pair of sealed ionization chambers in said housing, eachionization chamber comprising an elongated signal electrode, la sleeveelectrode surrounding the signal electrode, insulating sleeve meanssealingly connecting the ends of the sleeve electrode to the endportions of the signal electrode, a gaseous mixture of boron isotopesincluding the fluoride of B10 isotope in a relatively high proportion inone of the chambers, a gaseous mixture of `boron isotopes including theuon'de of B10 in a relatively low proportion in the other chamber, saidgaseous mixtures having substantially equal sensitivities to gammaradiation but having substantially different sensitivities to neutrons,a iilling of inert gas in sald main housing surrounding the ionizationchambers and isolated from the gaseous mixtures in the ionizationchambers, means applying high voltages of opposite polarity to saidsleeve electrodes, and means connecting the signal electrodes together,whereby the effects of gamma radiation on the chambers are substantiallycanceled, and whereby a resultant signal is provided at the connectedsignal electrodes in accordance with the difference between theresponses of the chambers to the presence of neutrons.

9. A neutron measuring device comprising a scaled main conductivehousing, a pair of sealed ionization chambers in said housing, eachionization chamber comprising an elongated signal electrode, a sleeveelectrode surrounding the signal electrode, insulating sleeve meanssealingly connecting the ends of the sleeve electrode to the endportions of the signal electrode, respective conductive guard sleevesmounted in the end portions of t-he chamber concentrically lbetween theinsulating sleeve means and the signal electrode and projectingsubstantial distances into the end portions of the sleeve electrode, andmeans electrically connecting the guard sleeves to the main housing, agaseous mixture of boron isotopes including the iiuoride of B10 isotopein `a relatively high proportion in one of the chambers, a gaseousmixture Y of boron isotopes including the liuoride of B10 in arelatively low proportion in the other chamber, said gaseous mixtureshaving substantially equal sensitivities to gamma radiation but havingsubstantially `different sensitivities to neutrons, a filling of inertgas in said main housing sur-y rounding the ionization chambers andisolated yirorn the gaseous mixtures in the ionization chambers, meansapplying high voltages of opposite polarities `to said sleeveelectrodes, and means connecting the signal electrodes together, wherebythe effects of gamma radiation on the chambers are substantiallycanceled, and whereby av resultant signal is p-rovided at the connectedsignal electrodes in accordance with the ditference between therespouses of the chambers to the presence of neutrons.

10. A neutron measuring ldevice comprising a sealed main conductivehousing, a pair of sealed ionization chambers in said housing, eachionization chamber* cornprising an elongated signal electrode, a sleeveelectrode surrounding the signal electrode, and insulating sleeve meanssealingly connecting the ends of the sleeve'electrode to the endportions of the signal electrode, first ionizable gaseous material inone of the chambers, second ionizable gaseous material in the otherchamber, said gaseous materials having substantially equal sensitivitiesto gamma radiation but having substantially different sensitivities toneutrons, a iilling of inert gas in said main housing surrounding theionization chambers and being isolated :from the gaseous materials inthe ionization chambers, respective terminal members connected to saidsleeve electrodes and adapted to be connected to different high voltagesources, land an output terminal connected to the signal electrodes,whereby a resultant signal is provided at said output terminal inaccordance with the dilierence between the responses of the chambers tothe presence of neutrons.

11. A neutron measuring device comprising a sealed main conductivehousing, a pair of sealed ionization chambers in said housing, eachionization chamber comprising an elongated signal electrode, a sleeveelectrode surrounding the signal electrode, insulating sleeve'meanssealingly connecting the ends of the sleeve electrode to the endportions of the signal electrode, respective conductive guard sleevesmounted in the end portions of the chamber concentrically lbetween theinsulatingY sleeve means andthe signal velectrode and projectingsubstantial distances into the end portions of the sleeve electrode, andmeans electrically connecting the guard sleeves to the main housing,first ionizable gaseous material in one of the chambers, secondionizable gaseous material in the other chamber, said gaseous materialshaving substantially equal sensitivities to gamma radiation but havingsubstantially different sensitivities to neutrons, respective terminalmembers connected to said sleeve electrodes, and an output terminalmember connected to the signal electrodes, whereby a signal is providedat the output terminal in accordance with the diierence between theresponses of the chambers to the presence of neutrons.

12. A neutron measuring device comprising a sealed main conductivehousing, a pair of sealed ionization chambers in said housing,'eachionization chamber comprising an elongated signal electrode, a sleeveelectrode surrounding the signal electrode, insulating sleeve meanssealingly connecting the ends of the sleeve electrode to the endportions of the signal electrode, respective conductive guard sleevesmounted in the end portions of the chamber concentrically between theinsulating sleeve means and the signal electrode and projectingsubstantial distances into the end portions of the sleeve electrode, andmeans electrically connecting the guard sleeves to the main housing,first ionizable gaseous material in one of the chambers, secondionizable gaseous material in the other chamber, said gaseous materialshaving substantially equal sensitivities to gamma radiation but havingsubstantially different sensitivities to neutrons, a tilling of .inertgas in said main housing surrounding the ionization chambers `andisolated from the gaseous materials in the ionization chambers,respective terminals connected to said sleeve electrodes and adapted tobe connected to sources of different high voltage, and an outputterminal member connected to the signal electrodes, whereby a resultantsignal is provided at the output terminal member in accordance with thediiierence between the responses of the chambers to the presence ofneutrons.

References Cited in the le of this patent UNITED STATES PATENTS OTHERREFERENCES Baum: Neutron Dosimetry, Atomic Energy Commission DocumentUR-381, March 29, 1955, pp. 74--77 relied on.

